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Space

Practical Gravity Shielding for Spacecraft? 230

Posted by Roblimo from the falling-off-the-leading-edge dept.
Anonymous Coward writes, "I saw this site today: 'The Gravitational Spacecraft from Fran De Aquino Warping to the deep space...' A researcher in Brazil says that it can be done. What do you think? Anyone working in this area care to comment? The site can be found here." Slashdot has no official editorial position on the feasibility of gravitationally shielded spacecraft, but if anyone wants to send us a review unit, we'll gladly put it through its paces and do a writeup about it.
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Practical Gravity Shielding for Spacecraft? More

Practical Gravity Shielding for Spacecraft?

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  • Well, ignoring the prospect of time travel (which the "scientist" in question does NOT pose as his motivation for making these calculations. I'll explain l8r), I'd still have to say that the guy is a nutter. True, it may be a THEORETICAL possiblity to negate gravity by shielding. But how will he in REALITY make a machine that not only holds ONE photon in a fixed postion relative to the machine, but an entire barrier of photons ? It's just not possible. You cannot get a photon to "stay"... These are not pets, they're not likely to do as they're told. And at any rate, if there's another physical reality INSIDE the "shield" (seeing as this would be unaffected by the forces of this universe), how would one define such basic terms as Velocity, or even distances relative to "Your" world. Nah.. This is just a load a cr*p, like the chicken and the egg, or the turtle and the rock and other such conundrums. Apply just the slightest logic to the arguments and they fall apart. As for the time travel issue: 1. I don't see the man claim to achieve such goals through the use of this technology, but on the other hand, if You create Your own little universe inside the shield, whos' to say that the laws of time would not be equally screwed up ? 2. Yes, i know that Gravity in extreme cases can attract light, thereby increasing/descreasing it's speed. But that doesn't mean that Gravity is the ONLY factor in theoretical timetravel.

  • Mass of a photon is not 0 (Score:1)

    by Anonymous Coward
    IF it were 0 then it would not be effected by gravity and solar cells wouldn't work. Also gravity shielding is impossible since mass itself warps time-space and removing mass is impossible. WIth an external force you can counter-react gravity by not shield it.

  • Let me clear up a few points... (Score:1)

    by Anonymous Coward
    Photons & mass: Ok, photons are made of energy, not matter. Classicaly speaking, they therefore can't have mass. However, in Quantum Mechanics, energy and mass are pretty much the same thing, just in different concentrations. It's been proven that photons have momentum, so they either have mass or energy. So I guess you could say that it's current mass is its momentum/c (velocity = c), and to get it's rest mass, divide that by infinity. (Or more accurately, find a number that when multiplied by infinity equals the photon's relativistic mass.) I'd like to shake the hand of the man who answers that one. Inertial frames of reference: It seems the discussion has moved away from this, but I'll mention it anyway. Einstein did say that there are no absolute inertial frames of reference. But there ARE absolute rotational frames of reference. Think of it this way: If you're in a sealed room with a pool table, can you tell by looking at the pool balls whether or not you're moving? Of course not. The cue ball will only react to an acceleration. Now, if the room is spinning, you'll see this on the pool table. Because everything other than the exact center of rotation is under constant acceleration. Issac Asimov's Psychohistory Also a forgotten item, but I like to hear myself talk. Someone here refered to it as a bunch of bullshit. Foundation was printed in the 30's. (I think) So to my knowledge, psychohistory is the first printed discussion of Chaos Theory. (Kind of a variation on thermodynamics.) Granted, the existance of individual people in positions of power proves that it can't be applied to people. But I'm still kind of impressed that he talked about a branch of science that wouldn't even have a name for several more decades. -Dave davester@provide.net

  • Re:This is utter nonscience! (Score:1)

    by Anonymous Coward
    yah, april 1st isn't for another couple of days. or is this a warmup?
  • See, here where I am, it's 12:21am and I've been working in the studio all evening. This means that my comprehension of complex physics equation is lacking a bit (it may also have to do with the fact that he's using some funny characters).

    Still, it seems completely feasible. If you can create whatever it is that makes that screensaver at the bottom, I'm sure you can push a spacecraft to superluminal velocities.

    /me returns to staring at pretty swirly image.
  • I know of a company that are going to make an anti gravity system that can lift 160 tonnes and hold it hovering in the air indefinitely.

    The system is called CargoLifter and it'll be used to transport extremely large and heavy loads thousands of miles by air rather than road.



  • From the start of the website:

    "As we know , the photon has null inertial
    mass (mi = 0 ) and it doesn't absorbs
    others photons (U = 0 ). So , if we put
    mi = 0 and U = 0 in Eq.(1.04) , the result
    is mg = 0 . Therefore photons have null
    gravitational mass."

    A little bit later, on the same website, we get this claim:

    "This means that any body inside the
    shield will have null gravitational mass
    with relation to the Universe."

    My answer: I don't think so.

    Although photons may not have detectable gravitational mass, somehow, gratitational pull DOES have effects on light.

    When lights from far away starts pass near huge stars and/or blackholes, the lights were bend somewhat.

    That is an indication that photons _ARE_ effected by gratitational pull after all.

  • How can you be the enemy of what you don't believe in? How many of you are enemies of Earth's other moon? (the delusional among you are excused from this discussion)
  • Can/does a photon even exist if it isn't moving? Aren't they basically just radio waves fairly high in frequency/short in wavelength? Isn't a non-moving wave a non-event with non-properties?
  • Pretty pictures, but he doesn't say, or even propose a way to build this "Gravitational Spacecraft". It works in theory, but thats like saying you could be anywhere, everywhere, anytime you want just by going as fast as the speed as light. When your able to buy a ticket, then it works.

    Of course I been know to propose even more crackpot ideas then this, so who am I to criticize(sp).

    Steve
  • Personally, I never trust these kind of articles until I've seen the application of theory in Mindstorm Legos.
  • For values of airplane==spinning, highly polished, top shaped thing.

    I cant find a url for the program anywhere, but IIRC it wasent light per se pushing the spinining top up, but the air around the laser impact point becoming superheated.

    And at the time of the news report that I saw, it wasent going up much further than 20m or so.

  • If the mark of a good April Fool's joke is the number of people who think it is serious, this one is a real winner.

    (Posting the joke 2 days early is a little tacky, though.)
  • ... them pictures gives this away as a hoax ...

    Any real scientist would rather spend time in the lab than creating pretty pictures. Just because you feel light headed doesn't mean that you've manipulated gravity.

  • Shielding gravitons with photons? First of all, all particles in the universe have SOME influnece of gravity and gravity can influence them in some way. Secondly gravitons have never ever ever been observed or proved with any real scrutiny, they're akin to WIMPs (weakly interacting massive particles) in that they are ideas and not much else. Tp null out gravity (in a way that has been proven) is to use two very large masses. If you're interested in the subject pick up a book on LaGrange's theorems. Maybe I should send this guy a copy...
  • hey.. personally i think the little "galaxy" java applet is cool enough it deserves to have Slashdot link to it all by itself. :)
    it deserves at LEAST quickie status. At any rate it's more relevant than the page it's attatched to.

    Then again, if the point of posting something on slashdot is to begin a discussion, Roblimo hit gold with this one. I've rarely seen a discussion so intelligent on slashdot. We need more theoretical physics flamewars here!
  • Your argument would be true, but he measures
    weight in [kg], i.e. he uses mass units.
    In SI system, weight would be measured in
    Newtons.
    FYI, I have a Masters degree in physics and am
    going for my PhD.
  • The rest mass of a photon is zero indeed. However, photons definitely carry energy, otherwise Sun wouldn't warm the Earth, lasers wouldn't burn and most of quantum mechanics would be impossible, because transition between different energy states would become unfavorable. Now we all know E=mc^2. Photons have E!=0, so their effective mass is non-zero. Indeed, light cannot escape black (sic!) holes because of this. In relativity, any energy clump bends time-space. Don't get hung up on the word mass (or weight, as the referred site seems to prefer the layman's language) - photons definitely bend universe, as would gravitons (if/when discovered). General relativity is highly non-linear. Live with it.
  • Yes, but what if it is not the light bending, it is space, and light is just following the path that space gives it. Beodd.
  • Well... just remember that all high-energy papers are preprints before they're published. 99% of the stuff on xxx really does get published and is meaningful science.

    But as you point out, it isn't peer-reviewed, and anything that shows up there needs to be taken with a serious grain of salt until it's checked.
  • Saying "light bends in a gravity field" is different from saying "light has mass."

    The bending effect is a distortion of space-time by the mass of whatever is causing it. For instance, our local star (yes, the bright glowing thing outside) is so massive that it actually warps the space-time surrounding it. When light passes through that altered space-time, it follows the contours of it. It's like threading a small wire through an uncooked elbow noodle - it's going to follow the curve. Whether the photons have mass or not is irrelevant to the issue; the photons will "curve" because the path they follow is curved.

    Smaller objects (planets, humans, cats, the new Massachusetts quarter) also warp space-time due to their masses. However, the latter three objects provided as an example have such a small mass that the space-time warping is negligable. You'll never notice it, nor will anybody else. It's so small it is irrelevant.

    As a result, the issue of whether photons having gravitational mass would now become: do photons warp space-time in their vicinity (even on infinitesimal scales)?

    I, personally, would say no. Then again, I'm not a theoretical physicist, so many answer may be akin to saying, "Yes, of course the Earth is flat."

  • You wrote: For example, the healthy blue glow *grin* seen in the water in fission reactors is due to electrons moving faster than light in the water.

    Oy vey.
    Lets try this one again... correctly this time...

    In water, with an index of refraction Nw (Nw=c/(velocity of light WITHIN water)), it is entirely possible that an electron exceeds the velocity of light WITHIN water (which is not c, but c/Nw). The (unhealthy) blue glow is Cerenkov radiation. The electrons do NOT go faster than c.

  • Yes, photons can be emitted or absorbed. Photons carry momentum. Momentum of a photon is well known E=p*c with p the momentum of the photon. If you really require a newtonian description, an absorbed or emitted photon can impart a change of momentum to the source of the photon. Having that absorption or emission occur over a time interval could show a nice delta_P over a delta_time which has dimensions of force (also called an impulse/impact).

    As for the rest of it, yes, you are wrong. Completely. Please review your copy of Sears, Zemansky, and Young, "University Physics" for more details.

  • It seems to me that the whole idea here is based on the assumption that gravity does not affect photons. This is an incorrect assumption. Photons ARE affected by gravity, this is theorized by General Relativity, AND it has been observed in real life. Basically GR says that as light passes near a gravity well (a planet) the light should be bent slightly. This implies that there has to be some sort of interaction between the photon and the graviton. So I don't think the gravitational sheilding would work.
  • Dude, pay closer attention in physics class.
    ...the word mass (or weight, as the referred site seems to prefer the layman's language)
    He's actually using the two words correctly. Weight is the amount of force gravity exerts on an object. He is specifically trying to negate weight, not mass. The mass is still there without gravitational attraction.
    Of course, he will have major problems getting the whole idea to work even if his theory is correct.
  • it's usually a pretty good sign that someone is saying something stupid.
    In this case, it's basically someone with an AOL account who is capitalizing on the confusing things that happen to mass and energy at relativistic speeds to convince some people that he's invented an antigravity ship.

    As many others have noted already, his assumptions about photons are a bit off (ie they don't have rest mass, but they do have four-momentum, which is what you really need to talk about anyway at that speed).

    Also, the "superluminal" bit goes on the assumption that since his ship is "anti-gravity" it's "massless" and therefore can go as fast as it darn well pleases. This is false in enough ways that I'm sure you can all come up with your own.

    Lastly, my impression of the overall idea is to use light energy to counteract gravity. Which is kind of like using rocket fuel to do the same, except it's much more efficient and we're no where near to possessing the technology to accomplish it.

    Anyway, that's my 100,000DM (in less than 20 minutes, even!)
    have a nice day
  • Actually, it *is* a particle in the sense of a proton/neutron! What exactly do you think a particle is?

    A particle is anything which carries energy or momentum. Period. In quantum mechanics, we can model vibrations of a lattice as collective excitations across the lattice ("normal modes").
    These collective excitations are called phonons, sometimes called quasi-particles. But they are particles - they are absorbed, emitted, recoil, and diffract.

    Quantum field theory (welcome to hell, boys and girls... enjoy renormalizations) is all about this - now, instead of some particles being collective excitations, *all* particles are
    collective excitations.

    This basically comes down to a question of exactly what is a particle, which is usually
    covered in a second-semester or first-year
    graduate course. Particle does have a sort
    of rigid meaning, but you can't revert to
    classical thinking all of the time.

    Plus you just have to get rid of that wave-particle duality crap. Waves are particles.
    Ever stand in an ocean? Ever get hit by a
    rather strong wave? (Not a crashing one,
    those are wierd... welcome to yet something
    else that physics can't describe yet) You
    feel it, don't you? But it's a *wave*, not
    a particle.

    Simple answer: collective excitations of "X"
    field where X is some quantum mechanical field
    are particles.

    Note that a graviton isn't a particle yet. We don't have a quantum mechanical gravity field yet. :)
  • Oh, I quite agree- however, if you look at these papers, they're a strong argument for some form of peer-review on preprints. :) My personal favorite is the derivation of the second law of thermodynamics... note it closely and you'll see that it didn't require anything in the article at all. (Not to mention that it's a bit strange anyway, saying "F=E" and then in the next sentence saying "F=E-TS")
  • Note to all Slashdotters:

    They are NOT papers. They are preprints. Anyone and his brother can submit a preprint. They have NOT been peer-reviewed (nor are they likely to EVER be peer-reviewed) nor have they been published at all (nor are they likely to be published at all) because they're totally bunk. :)

    xxx.lanl.gov (arxiv.org) preprints should not be taken as being fact. Hardly! Wait till you see something actually *published* and then you can take it with some conviction that it isn't crap science.

    Do not confuse a preprint with a published paper.
  • Summary: A photon has both energy and momentum, which are dependant on frequency. They may or may not be massless, but if they do have mass, it's less than 10e-45 kg (or somewhere in that vicinity, I don't know the exact upper limit we've calculated so far... it's small)

    Question: Why did you put a Kilo (10e3) on a gram unit that is so small? Are you trying to make it look even smaller?

    Quack

  • As far as i can tell the aol webpage is done by some antigravity fanboy, Jean Louis Naudin. The person who actually came up with all this crap is Fran De Aquino from Maranhao State University, Brazil. He has his own webpage at:
    http://www.elo.com.br/~deaquino/ [elo.com.br]
    He apparently has presented stuff in journals, at least according to the good folks at los alamos:
    http://eprints.lanl.gov/cgi-bin/w3vdkhgw?qryRDAD2_ .g_;phys-9904018 [lanl.gov]
    The references cite that he's been in the Electric Spacecraft Journal, i have no idea what sort of a publication this is though, it could be a trashy magazine about UFO abductions or what not parading as a journal, i'm not too sure.
  • Well, if a star does not generate enough photons to cancel gravity then this gadget would have to have a greater photon density than at the surface of a star. For that matter, what effects does he predict from photon-gravitron blockage; it would be nice to look for such reactions in an appropriate accelerator...but his physics isn't at that level.
  • Yes, but it is mentioned that the iron absorbed all of what was emitted and confirmed with an ELF meter. So he created EM pulses from the opposing antennas, with the pulses traveling in coils in opposing directions with opposing polarities, and thinks that the resulting photons cancelled the gravitational attraction to/from the iron within the iron, and he thinks there was no leakage.
  • Actually, Photons have a zero mass all the time. What they have is momentum... it's kind of counter-intuitive to someone used to thinking about a Newtonian universe where momentum = mass * velocity.

    Turns out that that's only part of the equation, you see. Bottom line, photons have no mass. They do have momentum. If you really want, I'l calculate it for you. :)
  • it already is a problem: this is why one of the mars probes kept going off course (the one with the unit conversion error) It only had one array of solar panels, so the solar wind produced an asymmetrical force (moment) on the probe and the gyros had to counteract it...
    I read this in the IEEE magazine a while back.
  • ...the /. story I needed to try and read after beer.

    --

  • Photons definitely have a nonzero stress tensor and as a result do produce gravity.

    Yup, thus explaining why halogen lamps always seem to catch any airborne object.

  • This reminds me of the book "How to Build a Flying Saucer" by Andrew Paulicki (not completely sure on the name, were is that darn book?). The math and some of the diagrams at the web site look damn close to the book.
  • now gravitational shielding. There must be something in the water.
  • Well, a photon has zero inertial rest mass. Which, since as far as we know, inertial mass=gravitational mass, would mean that a photon also has zero gravitational rest mass.

    The bending of light doesn't occur in Newton's world, anyway. You have to resort to general relativity, and there light does bend, even though it doesn't have mass. So this doesn't neccessarily invalidate what he's saying. Though I'm personally very skeptical.

    Of course, I am not a physicist.
  • ??? is certainly the correct choice of symbol there. Not only is there no centrifugal force, there is no centripital force.

    However, there is a centripetal force, which all the posters here seem to fail to understand. When you swing a ball around your head, it maintains its circular path because _you are applying force to it_ via tension in the rope. The name for this force which you are applying is 'centripetal force' , because your hand is at the centre of the circle and that's where the force stems from.

    If you stop applying this force, the ball (surprise surprise) continues to move in the direction it was moving when you stopped applying the force, and at the same velocity. Note that this corresponds to a tangent to the circular path it was moving on before.

    Now, this gives me an idea. Suppose this anti-gravity research is completed, and we have a portable device which can disable gravity locally (or even better, reduce its effect by a variable amount). When you engage this device, you would start to travel upwards (like when you release the ball, _as seen from the perspective of the ball_). Once you thought you were getting too high, you could re-engage your device and drop down a bit.
    Coupled with some direction-control jets, we have a portable flight device. No more paying air tickets and waiting in queues at the airport!

    And, to top it all off, this flight device could run a beowulf cluster of linux machines, and you could control its flight with GIMP. And, while you were passing the time, you could watch Natalie Portman pour hot grits down her pants! Perfect.

  • The superb sci-fi novel "The Mote in God's Eye", by Larry Niven and Jerry Pournelle, does in fact feature this. An alien society launches a probe which has a huge light parachute , and propels it for 35 light-years using gigantic ground-based lasers.
    (Until humans discover it and destroy it in paranoia... but that's for you to find out :) )


  • @@ how do you know?

    @@ have you seen a photon

    @@ i have not

    I suspect you have. I've seen billions of them.

    dylan_-


    --

  • Yes in fact we do know that photon's have gravitational mass in the sense that photons generate a gravitational field just like other matter.

    Yes photons have zero rest mass in the sense that a positive rest mass would result in infinite energy given that they travel at the speed of light. However photons are not in fact at rest and can never be at rest and do in fact have mass.

    That photons have mass is easy to deduce from their possesion of energy and the equivalence of mass and energy. Moreover they must produce a gravitational field as they are affected by gravitational fields (gravitational lensing) and so by conservation of momentum must in turn generate a gravitational field.
  • Now, the way gravity works (in graviton language) is that gravitons can be emitted or absorbed by anything with a nonvanishing stress tensor. (The bigger the stress tensor, the more easily this happens) So say two massive objects are moving along; Alice (having a nonzero stress) emits a graviton, and recoils by conservation of momentum. Bob (also having a nonzero stress) absorbs this graviton, and also recoils. So effectively both Alice and Bob have changed their momentum, i.e. exerted a force on one another.

    I'm hoping that someone can clarify this for me: I'm certainly not a Physics major, but if two particles come near each other and recoil from each other as a result of a graviton exchange, wouldn't that be a bit backwards from what actually happens? Namely, gravity is attractive (I think we all can agree on that!) and so it would seem that the absorption of a graviton would cause a particle to accelerate towards whence the graviton came.

    Now, in my silly little physics-naive head, the only way to make this work with Conservation of Momentum would be to postulate that gravitons have negative mass. Is this so? Just curious. Last I heard the existance of the graviton proper hadn't yet been affirmed by observation and in fact this whole bit about integrating quantum mechanics and gravity was still causing some fair number of very intelligent people to bang their head against very solid objects.

    I'd appreciate it if someone could screw my head on straight with regards to this: how does a graviton work? Might gravity actually not exhibit wave/particle duality? Please shake a clue stick in my direction.

    David E. Weekly [weekly.org]

  • Anyone remember the old Ray Harryhausen movie where the mad scientist painted gravity-blocking paint over the space capsue? An I can't find an IMDB link or similar, so here [cosmoweb.net]'s a private page about it.
  • I note from your formula that the gentleman's science reduces to BS. : )
  • They can't cancel each other out because the spin operators won't.
  • Why not do a review of something interesting?

    I recommend Robert Forward's "Indistinguishable from magic" for a start. At least his devices, as impossible as they seem, are based on real science that could potentially be built with a future technology.

    Regards,
    Ben
  • Seriously - bending the topic from anti or shielded gravity to communications - a real scientist wrote a piece of fiction which (iirc) implied that if gravity could be modulated we would be able to communicate at the speed of light squared. So, if one of a pair of objects could be made to appear to change mass, how long does it take for the other one to 'feel' the change in force??
  • ...since Man Friday has a rest mass of zero, the the Crusoe chip will achieve faster than light travel if you give it a really fresh cup of coffee...

    Seriously, any page that has the phrase "Free Energy" should never gain the honour of a /. story...

  • This story is to Greenglow as crop circles are to NASA's breakthrough propultion initiatives.
  • Anyone got a URL for the LightShip project? (A mirrored vehicle propelled by pulsed, groung-based lasers)
  • While slashdot can hardly be expected to be an expert in all areas of science, or to be a perfect, much less final, judge on the merits of specific scientific or pseudoscientific theories, I do think some common sense can be excersized in seperating speculative pseudo-science from hard science.

    Perhaps a pseudo-science category, with an appropriate x-files or flying saucer logo, would be in order for articles such as this, which are on (or beyond) the fringe of conventional science. Save the scientific icon and category for hard, well established scientific articles.

    This would IMHO boost slashdots credibility when publishing headlines for scientific articles, without censoring the more speculative (and often nonsensical) stories which, while having perhaps little scientific merit, do give some wonderful ideas for science fiction/fantasy stories...

    Of course, there would be the incessant "this belonged in the other category!" arguments, but at least a nominal effort to seperate the wheat from the chaff would be a net positive, IMHO.
  • May I suggest The Elegant Universe [barnesandnoble.com] by Brian R Greene?

    It offers an excellent, laypersons explaination for Special and General Relativity, Quantum Mechanics, String Theory, Superstring Theory, and M-Theory. It explains these theories and their ramifications in terms easilly understood, as well as their limitations.

    You may not like Quantum Theory (Einstein didn't like it either), but it has been demonstrated to be correct, as far as it goes, as has General Relativity.

    I too wish there was some way off this rock in our lifetimes, and there may well be if we get off our butts and build a space habitat or two. But, alas, unless M-Theory contains some interesting suprises for us over the next few decades (and it very well could), while we may make it into space, it is unlikely any of us will have the pleasure of walking beneath an alien sun, unless medical science delivers that immortality serum I've been asking for for years now, and we're willing to make a journey of hundreds or even thousands of years.
  • Ok, I basically do QM for a living, so I'll try and address your points:

    1. Well, they are, inasmuch as anything else is. A body in motion will tend to stay in motion. Other than that, they aren't perpetual motion machines, in the sense that you cannot get energy out of them indefinately, sooner or later they drop into their lowest energy state, and you can't get any enegy out of them (even if theydo still have a non-vanishing momentum)
    2. Not sure I see the diffrence. Every two particles with mass experiences an attractive force between them proportional to the product of the masses, and inversely proportional to the square of the distance between them. The force is towards the other particle, (this is easily demonstrated) so I'd guess you'd call it a "pull".
    3. This isn't a QM question, it's a relativity question (so is the last one, I guess), but I'll give it a shot anyway. It is the speed of light (in vaccum) that's constant. Multiple experiments have demonstrated this. A consequence of the effects that make the speed of light constant is that nothing can travel faster than light (also well demonstrated).
    4. This makes very little sense to me. Electrons are (mostly) conserved. (there are some processes that create/destroy electrons, but they all conserve charge, and/or other things related to charge). Why the universe has equal amounts of positive and negative charge is a somewhat open problem in science, but is a little deeper than your question goes...
    I guess I'll be like the rest of the geeks and keep wishing that some one will find some way to get off this rock we call home before I die of old age.
    heck me too, just this aint it. But the solar systems big 'nuff for my lifetime :)
  • Quantum physics and Newtonian physics. It sounds like you want the quantum world to be governed by the newtonian rules. I'm sure someone will correct me if I'm wrong, but that won't happen unless both fall under a Unified physics ruleset.

    If you want a good general idea of why quantum physics is so odd, it's cause stuff always gets weird at extremes.

    Bad Mojo
  • It's a bird!

    It's a plane!

    It's, it's... Why, it's an anti-gravity donut stack delivery system. The future is looking good for law enforcement! :)

    I like that java animation though. That's pretty cool. And probably the most technically impressive piece of the site.
  • Three comments:

    1) The website on AOL is someone paraphrasing De Aquino's work. De Aquino's own website isn't any better, but the papers in peer-review journals seem to be much better. Sometimes any of us can sound like a fool without a good editor.

    2) I'm not sure you're interpreting 'gravitational mass' as Aquino intended. Specifically, De Aquino has papers addressing addressing the (non?) equivalence of gravitational and inertial mass (e.g.) [arxiv.org] as an underpinning of his (her) theories.

    3) How do you reconcile the stress tensor and the type of gravitons you espouse (spin = 2, and therefore require quadrupolar transitions to be emitted) with the fact that gravitons must escape from black holes? It would seem that this type of graviton could not (as far as I can tell)? (Yes, gravitons interact with gravity. Hence the fact that gravitational fields have their own 'mass equivalence' induced gravitational fields.)

    __________

  • you talk about the two as if they were seperate things.. but in fact if you can control gravity you effectively have time travel cinched. If you take einstein's equations and make the variable for effective mass negative, you wind up with the time variable becoming negative as well. They said something like that in physics class. Or maybe that was when you're travelling faster than the speed of light..? Um, never mind. It isn't important. Trust me, i'm right. And if you don't believe me, i can make some pictures in KPT Bryce for you that will prove my correctness beyond a doubt.

    Anyway, it is well known that there is a definate link between gravity-based travel and time distortions. The government scientists at Area 51 have been doing extensive testing and refining of aircraft with a gravity-based engine based on technology recovered from crashed alien spacecraft. It has been well documented that exposure to such technology frequently results in "lost time"-- things in the general area of the gravity engine temporarily move at a faster rate through time than the world around them, causing them to wind up having short amounts of time "disappear". We know this information, by the way, because agent Fox Mulder of the FBI was able to infiltrate Area 51 after the reality warping in the general vicinity after a gravity-based spacecraft malfunctioned overhead his car on the highway outside area 51 and crashed caused, causing Mulder and a high-ranking Area 51 official to temporarily swap minds in an episode spanning two weeks. As a result we were able to get much information about this fascinating technology that is directed gravity propulsion.

    -mcc-baka
    INTELLECTUAL PROPERTY IS THEFT
  • (1) Very possibly. But the theory itself is still cracked.

    (2) Nonetheless, de Aquino claims that the null mass of the photon (the vanishing of its inertial rest mass) implies that it is outside the effects of gravity. I argue that the way de Aquino referred to the mass of the photon as having to do with gravity is simply wrong.

    (3) This is a good question and one that bugged me for a long time. The real answer is that the graviton picture - the approximation that the gravitational field can be decomposed into small excitations which behave more or less like free particles, with some additional interactions - is only valid in the limit of weak gravitational fields. For black holes and similar strong fields, the self-interactions of the gravitons are so strong that they can no longer even approximately be thought of as particles.

    This actually is fairly closely associated with why quantum theories of gravity are hard. Basically one can show that a quantum field theory of spin-2 particles is inconsistent. It reduces to something consistent in the low-energy approximation, where gravity can be thought of as the exchange of individual gravitons. But as soon as the self-interactions of gravitons becomes significant, this picture of gravitons as particles stops producing meaningful predictions.

    And this is exactly the point where we have to turn to our one well-defined and reasonably well-understood theory of quantum gravity: superstring theory. While this theory is not experimentally verified, there is substantial reason to believe it will be. (Which I won't go into now :) Most importantly, string theory can correctly predict aspects of the low-energy behavior of black holes which show up when classical GR starts to fail. (Hawking's "information paradox" is such an issue)

    So it's possible to calculate the gravitational properties of black holes using string theory and then back away and say: If I'm far away from the black hole, it just looks like an ordinary gravitating object, gravity is weak, and the graviton approximation should apply. Indeed it does; it turns out that, from far away, the black hole looks like a uniform sphere of radius equal to its event horizon, and as far as a distant observer is concerned, it does indeed emit gravitons from this distance. But all information about the internal structure of the black hole is kept within its event horizon; we can't see inside the horizon using these gravitons.

    Yonatan
  • You are describing a radiometer, and the reason a radiometer spins is due to the black/white surfaces. Light hitting the black surface is absorbed, heating the air next to it, and causing an imbalance. It is not due to any kind of 'photonic pressure'.
  • Recall that his intent is not that radiated photons act as a propellant, but that they negate graivty, acting as a shield, to block gravitons.
  • I am not nearly as well educated on the subject as the previous poster, and I hope he answers your question.

    I believe the answer, however, stems from the fact that 'particle' is not an absolute term.

    As he mentioned, it can be viewed that a photon is just a localized distortion of the EM field. It is a quanta that an EM wave can be broken down into. It is not necessarily a 'particle' like an atom or a proton/neutron, though it may cause some similar effects in the universe.
    The same can be said of gravitons. It is not as if the object spits out a proton or something, and propells itself away. The graviton is a localized distortion of spacetime. The choice of the word 'recoil' is probably just misplaced.

    I could muse that we could view this as 'spacetime' becoming more 'dense' between object, as gravitons are emitted, causing a percieved attraction.. but I'd probably be way off base.
  • http://linquirer.tesla.cx
  • So what exactly is the speed of gravity? I've always wondered about that one?
  • If you look at the author's SystemG page [aol.com] you see that he's done some experiments.

    For some reason he's not using photons, he's using electrons in a 60HZ dual coil [aol.com] (two ELF antennas in a coil -- look at the diagrams and the coils are only connected at one end).

    He doesn't describe the kind of table, room construction, or anything else which eliminates magnetic effects. Nor whether the materials might bend or expand when heated, causing the coil to lift itself by bending the input leads (and hundreds of amps should cause some heating...).

  • If you look at the links and credits in the pages, you end up with the JLN Labs [go.to] page. Full of other interesting "technology" tips.
  • You are smarter than that.

    And there's another for the "Crackpot Theories" department. ;)

    Seriously, he's probably just bored and wanted to watch all the knees jerking.
  • (2) Nonetheless, de Aquino claims that the null mass of the photon (the vanishing of its inertial rest mass) implies that it is outside the effects of gravity. I argue that the way de Aquino referred to the mass of the photon as having to do with gravity is simply wrong.

    As might be evidenced by one or two "black holes" out there...

  • So what exactly is the speed of gravity? I've always wondered about that one?

    The speed of light, or slightly slower; gravitational waves are governed by similar constraints to the permittivity/permissivity constraints that govern light in normal space (ie. same medium, different units).

    Simon
  • I find this kind of story entertaining in a sick sort of way but my take on all these crackpots is that someday one of them will be right and they will get ignored. We've seen it before and well see it again.

    I've read about quantum physics since I was about 10. I still don't buy it. I love Dr Feynman's books on sub particle physics made easy. One part I like the best is he makes a statement that any physical law that is complicated in the past has been untrue (early chemistry, astronomy, etc) but then goes straight into some of the most bizarre stuff in modern science. I think he was one of the few scientist that would have gladly thrown away all they knew about a subject if it was replaced by something simple and correct.

    My basic problems with modern quantum physics include:
    1) Electrons seem to be perpetual motion machines.
    2) Does gravity push or pull and how would one prove it? If it pushes that would sure explain why all the Zero G crystal can be duplicated on earth.
    3) Speed of light seems to lock down lots of physics. Is it the speed of light or the speed of gravity that is the limiting factor?
    4) There always seems to be enough electrons.

    I guess I'll be like the rest of the geeks and keep wishing that some one will find some way to get off this rock we call home before I die of old age.
  • Momentum, energy, mass and so on get a bit mixed up once you start dealing with special relativity.

    In short, photons have no rest mass. However, they do have both momentum and energy, as those two are very much related in special relativity. Anything with mass or kinetic energy has momentum.

    Therefore, being hit by a photon does give you a push, and shooting a photon off does make you recoil. Though not much. This is the principle behind light sails.
  • Here's a quick calculation of the power needed to run a 100% efficient spacecraft near the Earth (warning: very rough. =)
    E=.5mv^2=.5m(10t)^2=50mt^2. (rounding, etc).
    So in one second, E=50m=500,000J for a 10,000kg spacecraft... Thus, at LEAST 500,000J/s=500,000W must be used to counter the falling spacecraft, by basic conservation of energy.... That's 5,000 100W lightbulbs... This is going to be a BRIGHT spacecraft.
    Where is he going to get all this energy?
    PLEASE correct any wrong assumptions/simplifications... Just throwing this out there....
  • Yes, but it would take an infinite amount of energy to counteract the mass-increasing effects of lightspeed, so we're right back where we started.
  • On the second reading, I see that the guy does propose to use this as a source of free energy.

    Move along, citizens. Nothing to see here. Just another inventor of perpetuum mobile.
    --

  • I think I caught the first booboo he made. Photons *do* have a rest mass of zero. But uh, they tend to travel at the speed of light relative to all frames of reference (or so I hear :). Because of some relativistic quirks, this gives them a slight mass.
  • If we were to shield ourselves from gravity, while on earth, we would not float into the air! We would be flung from the earth by centripital force, not unlike having a ball at the end of a rope and twirling it around my head and then letting go.

    Consider the speed at which the earth is spinning AND the speed at which the earth is revolving around the sun (about 18000miles/second).

  • Yes I realize how a TV works. How it works
    makes perfect sense. A big coil that influences
    the path of a stream of electrons.

    I never said coild don't have uses. I just have
    doubts about their ability to "nullify gravity".

    Seriously...he is talking about photons...nearest
    I can figure he plans to just shove so much
    current through the wires that they glow like a
    light bulb....then why use insulated wire? Where
    will the light go?

    Certainly that much heat (if thats what he
    is doing) would burn righ tthough any
    insulation I know of...furthermore the high
    temperature would cause the metal to oxidise
    very readily in normal atmosphere.

    Yes it "seems bogus". No thats not a logical
    statment. Its a gut feeling based on presentation,
    and the fact that AFAIK the basic premise is
    flawed. (not to mention that people have been
    playing with putting huge currents into coils
    of wire for a long time and mysteriosluy noone
    has noticed this ability of theirs to
    "nullify gravity".

  • correct and incorrect.

    radiometers spin because the light is absorbed
    by the black surface....which heats it up. Thus
    when air (there is a small amount of air still in
    the glass tube...very low pressutre) strikes the
    black surface, it is heated and moves away with
    greater force,...thus pushing on the black side
    more than the cooler white side.

    In the low air pressure (and thus low air friction
    ) environment...this is enough to make it spin.

    However....

    If you pump out even more air...thus lowering
    the air pressure even more...this effect becomes
    much less...and the fins actually DO spin in
    the opposite direction.
  • Okay, I really don't want to sound like flamebait here, so I'll be gentle ... :)

    I understand that those posting these news stories can't have full background knowledge on every bit of science involved, and that some things will sound good until the message board quickly fills up with what turn out to be really obvious flaws in the 'article'. Even so, shouldn't this have been taken with a grain of salt? An AC submits the story ($10 says it was the authors of the website), it's nothing more than a concept writeup on a user homepage, it has not even a GLIMMER of scientific scrutiny (doesn't come through scientific-news channels, no sign of it being submitted to any journals or universities or the like) ... the obvious prejudice of the AOL account isn't even NEEDED here.

    Sure, it's possible that someone will come up witha revolutionary scientific breakthrough independantly, and publish it on their web site before the scientific community can applaud it. But what are the odds? If you really want to put up stories like this, go ahead, but don't pretend this is real science. Entertaining sci-fi, sure ... but c'mon, light being affected by gravity is knowledge just BARELY above high school physics.

  • Gravity Shielding, Anti-Gravity a while Back. Am I the only one to see the conspiracy going on here? Its an attempt to draw all the LINUX geeks into space on 'ISS Linux' and have the 'ISS Enterprise' shot it down?

  • Well, I am a physicist, and I don't have a clue what this guy is arguing....

    It looks like he is manipulating formulae in an odd manner...

    And it looks like he is glossing over most, if not all, of General Relativity when it comes to spacetime, tensors, etc.

    Even so, where do these photons come from? At what frequency do they have to be? At what power? Probably be so intense they'd ionize, if not completely shred, every atom nearby, including you and your spaceship. And he talks about cross-sections between the photon and graviton... Talk about mixing relativity and quantum mechanics! Not good...

    At some level, this is all trickery, like Cold Fusion. Most physicists knew immediately that Pons and Fleishmann did not produce the level of neutrons they claimed, because if they had they would have been dead.

    Now, I have heard a talk about negative matter (and negative anti-matter) which react like (-m) instead of (+m) in F=ma and F=GMm/r^2. But when a (-m) piece of matter runs into (+m) matter, they anihilate leaving E=0 ! Maybe the guy needs some of this stuff to build his shield. Dunno how tho... Even odder negative matter generates negative photons ...

  • zunger-san is bang on correct. Thank you for submitting a nicely reasoned bit o' physics. Now if I could only think that clearly at 1 AM!

    hehe

    Eric

  • Unfortunately for the poor crackpot - there are quite a few problems with his physics.

    1) Photons _do_ interact. (Maxwells equations are linear - but theses are only an approximation of reality.) If you use relativistic quantum mechanics - you will find that there is a fourth order diagram where two photons can interact via the exchange of electron / positron pairs. This means that his parameter U is not zero as he claims.

    2) Photons do have zero rest mass - however photons contain energy (E=hf) This means that they contribute to the Stress-Energy Tensor T_ab

    Now G_ab = 8Pi T_ab Where G_ab describes the curvature of space-time. This means that if you have alot of photons - you can curve space. This means that his approximation of zero graviational mass for photons is also wrong.

    3) For his machine to work - it needs to radiate photons. Now this costs energy. I don't think he has calculated the amount required ;-). (Light sails accelerate extremely slowly - and this guy wants to make something accelerate at 9.8m/s^2) Oops.

    There is one final hint that something is wrong. At the bottom of the page are the plans for a perpetual motion machine - always a tip off....
  • I never took any physics, so I probably suffer from understanding shit from a Newtonian only train of thought, but the way I understand it, a photon has no mass or momentum, only direction and (?)speed, being 100% kinetic in substance, 0% potential.

    Am I right/wrong, I'm guessing anyway ;)
  • The momentum formula p=mv and many other classic formulas only apply to non-relativisic velocities, generally meaning they'll give acceptable answers when v ~0,1c (10% of light speed).

    The relativisic expression for momentum (p) is mv * gamma, where gamma is something called the Lorentz factor, (1-(v/c)^2)^(-1/2) //ASCII algebra suck, I know
    If you insert the values for a photon, m=0 and v=c, you get p=0/0.

    As all who have studied higher math whould know, the answer 0/0 doesn't really say us much, it could be virtually anything. 0/0 usually means we have to use other methods to find our answer.

    To make a long (algebraic) story short, the formula for a photon's momentum is p=E/c, where E is the photon's energy. E is expressed by hf (h=Planck's constant, f=the lights frequency), thus giving us p=hf/c or p=h/lambda(=wavelength)

    Dixi
    ---
    It seems slashdot.org is slashdotted all the time
  • A poster in a previous article was right. Slashdot has achieved the quality of weekly world news. The submissions often have spelling and grammatical errors, and I think pretty much everyone, however idle they may be, has something better to do than look at this tosh. Maybe if I write up a dodgy page about free energy machines using some error ridden maths (remember folks, often mathematical errors are difficult to find), and explain how men in black helicopters are stopping me from saving the world with it, I'll get slashdotted. Of course, these men in black helicopters are hired by the EVIL M$, who the Slashdot crowd ridiculously seem to believe to be worse than mining companies, agri-business, banks, and military-industrial complex corporations.
  • IANAPP (I Am Not A Professional Physicist), but I believe I can answer some of your questions.

    > 1) Electrons seem to be perpetual motion machines.

    Not really, it only seems that way. Entropy (slow loss of energy due to friction, gravity, magnetic fields and other elements opposing the current state) does happen, but is not observable in any way that a human would be able to notice, except for mathematically. The electromagnetic interaction between a proton and an electron provides the centripetal force necessary to maintain the electron in a stable orbit. The amount of gravitational attraction generated by an electron is so small that the orbit will not decay naturally for billions of years. Note that in this case, when the orbit decays, instead of falling inward toward the proton, like a spaceship around a planet, the electron will actually fly away from the proton.

    2) Does gravity push or pull and how would one prove it? If it pushes that would sure explain why all the Zero G crystal can be duplicated on earth.

    All particles push on objects they collide with, however in the case of gravitons, my understanding is that they push on the fabric of space-time rather than the physical objects. This could be seen as a mathematical hack, though, since in effect this is the same as the object itself warping space-time, as per Einstein.

    3) Speed of light seems to lock down lots of physics. Is it the speed of light or the speed of gravity that is the limiting factor?

    It seems to be the speed of light. And it's not really a lock-down, just a natural limiting factor in many equations. In a hyperbolic formula, one could say the asymtopes of the equation are limiting factors in much the same way.

    4) There always seems to be enough electrons.

    I don't think I understand the question here :)
    Enough electrons for what? There are atoms with fewer electrons than protons - they're called ions. You could just as well say there always seem to be enough protons, or gluons, or quarks, or photons, etc., etc. It doesn't mean anything.
  • In the future, it would be nice if Slashdot authors quietly discarded any "revolutionary physics" that have been posted up on web pages for some 2 years yet never mentioned in any scientific journals.

    Think about it. Do you really think Slashdot is going to be the place where the Real anti-gravity breakthrough is going to hit first (esp. since this "research" is claimed to have taken place YEARS ago)? How many of these crappy web pages do we have to see posted on Slashdot before we figure out the pattern?

    If nothing else, search for the phrase "free energy". That should be an immediate and conclusive clue that this article might not be all it's cracked up to be.

  • Re:Interesting idea... (Score:3)

    by drudd ( 43032 ) on Tuesday March 28, 2000 @09:58PM (#1164025)
    Not quite. A photon's momentum is related to its wavelength by the following relation:

    p = h / lambda

    so (p^2 * c^2) - E^2 = -m^2 * c^4 =>

    ((h^2 * c^2 ) / lambda^2) - E^2 = -m^2 * c^4

    c / lambda = frequency, so:

    h^2 * frequency ^2 = -m^2 * c^4

    and h * frequency = energy (for photon)

    so E^2 - E^2 = -m^2 * c^4, and c != 0,
    so m must equal 0.

    Unless you can demonstrate that either
    h*frequency != Energy for photon
    or h / lambda != momentum, both of which
    have been established experimentally, you
    cannot say a photon has mass (also see the logic in my previous reply to another poster).

    Doug

  • Interesting idea... (Score:3)

    by NOC_Monkey ( 73018 ) on Tuesday March 28, 2000 @09:35PM (#1164026)
    At first, this looks like a silly idea, using directed photons to counteract gravity. But, as shown by Einstein's mass-energy equivalence equation (E=mc^2), but expanded to include an object's momentum (p):

    (p^2 * c^2)-E^2=-m^2 * c^4

    Then we solve it for mass:

    -(SQRT((p^2 * c^2)-E^2)/c^4)=m

    Shows that a highly energetic photon _can_ have some mass. The problem is that the energy need is so high that any really significant change in the local measure of (g) would take enormous amounts of energy, most likely needing a direct-conversion reactor to genereate any usable mass. To give some scale to this, the fission of a single U-235 nucleus gives off enough energy to set off a mousetrap. And remember, a fissioned nucleus is not completely consumed. Only a small fraction of the mass is converted into energy. For a device like this to work, we would need to generate more energy than has been produced throughout all of human civilisation.

    Nice use of Bryce, tho...

  • This doesn't make any sense. (Score:3)

    by anatoli ( 74215 ) on Wednesday March 29, 2000 @01:02AM (#1164027) Homepage
    I mean, energy-wise. Let's consider this picture.

    BS (Fig.1)

    where B is a gravitating body and S is a grav-shielded spacecraft.

    As the spacecraft has zero mass, it has zero potential energy with respect to the body. Now let's switch our shields off.

    What happens? The spacecraft starts falling.

    B<---S (Fig. 2)

    This is because the spacecraft instantly acquires potential energy. This energy can be utilized when the spacecraft falls onto the body and emits lots of heat upon impact.

    B*S (Fig.3)

    (* signifies the great thermal explosion.) Let's switch our shields on again (presuming we've survived the impact). The craft starts to float away from the body.

    Switch them off, we fall and generate more heat. On again, float away. Off again, falling, generating heat. Cycle through Fig. 1--3 ad libitum.

    What we have is a perpetuum mobile of the first kind. Unless the energy required to switch the shields on and off is more than the energy we can generate. In this latter case, the shield is useless (what potential energy do you have with respect to the Andromeda galaxy?)

    Note how we don't care about the exact means of establishing the gravitational shield. Use any method. Energy conservation means you cannot win.

    Of course there's a smallish possibility that energy conservation does not hold. In which case we're better off fitting our cars and homes with perpetuum mobile. This has much more sense than exploring distant worlds.
    --

  • Its a big....hmmm (Score:3)

    by TheCarp ( 96830 ) <sjc.carpanet@net> on Tuesday March 28, 2000 @09:51PM (#1164028) Homepage
    Ok...basically he has 2 coils next to eachother.

    Look at the diagrams...thats all it is. A big
    metal tube with a couple of wire loops inside...
    looks like 2 or 3 turns each.

    Now....he claims to be throwing 300 A into
    this sucker...depending on voltage...thats a
    shitload of juice. Perhaps he is producing a
    magnetic feild strong enough, and properly
    oriented so that the coil is actually being
    suspended above the electronics in the scale
    below it?

    I dunno...it all seems really bogus. AFAIK the
    generally accepted theories of Gravity state that
    Mass warps space in 3 dimensions. So light is
    effected by gravity only because the space it
    is taveling thorough is bent (ie it follows
    a straight line in a curved space).

    Personally...I am very quick to call this pure
    bunk. Its just too pretty. I notice he claims it
    has been tested...yet there are no pictures of
    the actual device...just diagrams.

    It should also be mentioned that just because the
    math works, doesn't mean it physically works.
    You can play with math and make a good case for
    "white holes" (ie the opposite of a black hole..
    it spews out matter and never takes any in)
    however...its just because the math works both
    ways...there is no evidence that a white hole
    would actually exist.

    Perhaps this guy is just a crackpot...I would be
    interested to see more evidence myself. Another
    criticizm is...where are the photons comming
    from? He is talking about radiation etc...yet
    all he is really doing is dumping ALOT of current
    through a couple of coils that are near eachother.

    I am definitly skeptical.

  • Re:A Question on Details (Score:3)

    by spiralx ( 97066 ) on Wednesday March 29, 2000 @01:58AM (#1164029)

    ... but if two particles come near each other and recoil from each other as a result of a graviton exchange, wouldn't that be a bit backwards from what actually happens? Namely, gravity is attractive (I think we all can agree on that!) and so it would seem that the absorption of a graviton would cause a particle to accelerate towards whence the graviton came.

    The analogy of a particle emitting a force particle such as a gravition or photon, recoiling and then another particle absorbing the force particle and also recoiling works in the case of a repulsive force but falls down in the case of an attractive one.

    What you need to remember is that in quantum field theory force carrying particles (bosons) are just excitations of the underlying field - a photon is just an excitation of the underlying EM field. As such, when considering interactions like this you really have to consider it in the full sense of quantum field theory rather than simply treating them as particles, and seeing as how in this case the graviton would be "virtual" I doubt little things like the Conservation of Momentum apply :)

    Of course this is all in theory, since we don't have a quantum field theory of gravity. From recent developments it appears as though it will require a lot of unusual features, but IMHO it will at low energies approximate to a similar process as the other field theories.

  • SKAM! (Score:3)

    by BWS ( 104239 ) <swang@cs.dal.ca> on Tuesday March 28, 2000 @09:26PM (#1164030)
    Is it me or is it rather courious that these so called scientists have an AOL web page? I mean if they are reputible at all, won't they have a web page URL that either:

    1] belongs to a commerical company
    2] a university

    but AOL? camon, no smart people comes out of AOL. no self respecting scientist would publish stuff on AOL.

    Any crack pot [with some physics courses] can generate some mumble jumble drawing and formula and stuff. I won't belive it till its published in any reputible Science/Physics Journal [no, not the Playboy Science Section. The reputible journals have people that will analysis it and only publish the ones that are acutally true or promising, not any mumbe jumble. With the web, you can publish anything.

    BOTTOM LINE: wait till its published in a scientific journal, that way its gone through some peer review and we can be sure its not all crap!

  • Re:Mass of proton = 0? (Score:4)

    by drudd ( 43032 ) on Tuesday March 28, 2000 @09:31PM (#1164031)
    Not proton, PHOTON.

    Photons are massless, and do not exert gravitational force. Light does respond to gravity, however, since it is just following the shortest path of a deformed fabric of space.

    Photons have to be massless, otherwise they would have infinite energy:

    E = mc^2 * gamma

    gamma = 1 / sqrt( 1 - (v^2 / c ^ 2))

    insert c for v, and you get a zero in the denominator, thus any particle which travels at c must be massless.

    Doug

  • Re:Mass of proton = 0? (Score:5)

    by Anonymous Coward on Wednesday March 29, 2000 @02:36AM (#1164032)
    Almost correct. Photons have zero rest mass, but they also have energy. The deformation of spacetime in general relativity is measured by the stress-energy tensor, to which photons may contribute. So photons do exert a gravitational force. -- just another astrophysicist reading Slashdot instead of reducing his Keck data

  • Not quite. (Score:5)

    by zunger ( 17731 ) on Tuesday March 28, 2000 @10:02PM (#1164033)
    OK, some physics perspective on this...

    One-line summary: These guys are better at graphics than they are at physics. No, it doesn't work.

    Since the discussion below is a bit more technical than I usually send to Slashdot, here's a brief summary:

    * No, it doesn't work. There are in fact some very fundamental reasons why it can't be made to work, either.

    * It can be shown (through some fairly messy math) that it is not possible to cancel gravity by introducing other particles, unless you somehow had negative-mass particles to strap to your spaceship.

    * Even if you could do all of this, how would you hold a sphere of photons in place?!

    The rest of this is a bit heavy, so you may just want to skim. If you want to get a fairly easy-to-manage and good intro to this topic, you may want to check out Rindler's _Introduction to Relativity_.

    Okay, on with the show. There are three basic problems with this suggestion.

    Problem 1: "Photons have a null gravitational mass." It is true that the rest mass of a photon is zero. (The rest mass of a particle is the mass of that particle as measured by an observer at rest with respect to it. Special relativity tells us that objects with nonzero rest mass must travel slower than light, and objects with zero rest mass travel exactly at the speed of light.) However, the "effective mass" of an object for gravity purposes is <i>not</i> its rest mass. The quantity which generates gravitational fields is a more complicated quantity called the stress tensor, which is nonzero any time there is energy or momentum in a system. Photons definitely have a nonzero stress tensor and as a result do produce gravity.

    In fact, because of the way the stress tensor is defined, it is impossible to entirely cancel it with <i>any</i> configuration of masses, unless you somehow had something with a negative mass. If you find something like this please let the rest of us know; there are several well-known, mathematically valid, ways to design superluminal drives based on them. However there are also several results that quantum mechanics prevents the formation of more than infinitesimal quantities of such matter.

    So that's problem one; photons don't have "null mass" (whatever that means) and so don't automatically cancel gravity. On to

    Problem 2: If we had a shell of photons around our ship, they could exactly cancel the gravitons. Not quite. First of all, I should say that gravitons are the smallest quanta of gravity and are in many ways analogous to photons. (In the language of general relativity, where gravity is a stretching of spacetime, a graviton represents a localized deformation of spacetime. Similarly, a photon represents a local deformation of the electromagnetic field, which is the particle physics way of stating the fact that light is an electromagnetic wave.)

    Now, the way gravity works (in graviton language) is that gravitons can be emitted or absorbed by anything with a nonvanishing stress tensor. (The bigger the stress tensor, the more easily this happens) So say two massive objects are moving along; Alice (having a nonzero stress) emits a graviton, and recoils by conservation of momentum. Bob (also having a nonzero stress) absorbs this graviton, and also recoils. So effectively both Alice and Bob have changed their momentum, i.e. exerted a force on one another.

    Now, say I wanted to cancel this out with some shell of Mystery Particles which would give the exactly opposite forces. These particles would have to do a couple of things:

    * First of all, they had better travel at the speed of light so that they arrive at the same time as the gravitons in order to cancel them out. So these had better be massless. (See note above)

    * Second, they should be able to interact with any particle that has mass, and they should interact equally strongly with any two particles of the same mass. (e.g., if our mystery particle hit electrons twice as hard per unit mass as they did protons, they wouldn't just cancel the gravity, they would also knock about everybody's particles quite a bit.)

    Now it turns out (and this would require some lengthy tangents to explain in detail) that these two conditions are enough to <i>uniquely</i> specify the properties of this particle! In particular, the particle must have zero mass, and (for those of you with some physics background) have spin 2 and be symmetric tensor fields. These are exactly the same as the properties of the graviton, which isn't surprising since these things have to cancel them. But...

    * It's fairly straightforward to show (using a bit of field theory) that forces mediated by spin-2 symmetric particles are always attractive. (Like gravity, and as opposed to electromagnetism. EM is mediated by photons, which have spin 1 and can be either attractive [opposite charges] or repulsive. [like charges]) So unfortunately, no Mystery Particles can cancel out gravity.

    OK, and just in case this isn't enough, one other point:

    Problem 3: Even if you had some magic way of getting around this, e.g. introducing multiple species of graviton and giving them some extra interactions which let them exactly cancel gravity while somehow not affecting the ordinary operation of gravity in <i>any way</i>, (Which, incidentally, doesn't work; if these particles are just like gravitons, they should be produced in nature just like gravitons and so would be everywhere) How the hell are you going to keep a bunch of massless particles sitting in a perfect shell? They're <i>massless</i>. They tend to fly away.

    (OK, and now a more technical note for those of you who bothered to read this far: There is actually one other possible way to satisfy the constraints for a mystery particle, allowing it to couple to matter identically to gravitons without also being a graviton. Namely, the particles can be part of a multiplet of particles which are related to gravitons by some continuous symmetry [discrete symmetries would just give multiple graviton species] and so all matter would have to couple universally to them as well. This could be done even for particles that did not have spin 2.

    But some math prevents this from working. If we want our mystery particle not to have spin 2, the symmetry must relate particles of different spin. By a key result known as the Coleman-Mandula Theorem, the only way to achieve this is with a type of symmetry known as supersymmetry. If you do this, you find that it is possible to construct a complete suite of particles called a "gravity multiplet," consisting of the graviton and several particles with spins ranging from zero to 3/2, all of which couple universally to matter and do everything we wanted.

    So does this solve it? Well, not quite. The condition for these supermultiplets to cancel gravity is very well-known; it's called the BPS condition, and it says that the objects exerting the forces on one another have to obey very specific relationships between their charges (electric and other) and their masses, as well as some requirements about their relative shapes and orientations. When you work it all out, there are a few very specific configurations where this happens, but (1) It doesn't happen for arbitrarily shaped objects like spacecraft, and (2) All of this would require that supersymmetry be present and unbroken in nature. This would have some <i>really</i> visible experimental consequences, e.g. atoms looking pretty much nothing like they actually do. Such a symmetry is quite definitely ruled out.

    So I'm afraid that there's nothing of this sort that can cancel out gravity. Sorry...

    Yonatan

  • This is utter nonscience! (Score:5)

    by morris57 ( 23356 ) on Tuesday March 28, 2000 @09:47PM (#1164034) Homepage Journal
    Rob, Jeff, Roblimo, Andover, VA:

    You guys have lots of money now. Hire a freaking science consultant already. A responsible site that claims to be "News" does not put this story under the category of "Space" (thankfully it was not posted under "Science"). Put it under "Humor" or "Crackpot Theories".

    It's time to realize that there is a lot of bogus stuff out there. Please don't be fooled. You are smarter than that.

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